Grand Rapids
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grand Rapids clear sky chart from 2005-11-06 to 2024-03-28.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
34%
24921
13%
81090
9%
24818
39%
24818
16%
59635

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 10thNight
17%
945
5%
4407
3%
926
21%
926
5%
3770
January11th to 20thNight
16%
946
3%
4428
2%
930
19%
930
4%
3986
January21st to endNight
18%
1039
3%
4875
2%
1023
21%
1023
4%
4306
February1st to 10thNight
18%
789
8%
2433
4%
789
22%
789
8%
1996
February11th to 20thNight
19%
740
11%
2219
5%
740
25%
740
11%
1750
February21st to endNight
16%
628
10%
1884
2%
628
23%
628
12%
1450
March1st to 10thNight
27%
760
15%
2280
9%
760
33%
760
18%
1610
March11th to 20thNight
28%
760
20%
2151
9%
760
37%
760
24%
1434
March21st to endNight
28%
757
16%
2192
9%
757
36%
757
19%
1572
April1st to 10thNight
26%
540
13%
1800
9%
540
33%
540
18%
1243
April11th to 20thNight
30%
533
19%
1723
11%
533
41%
533
20%
1160
April21st to endNight
33%
540
17%
1620
9%
540
40%
540
23%
1081
May1st to 10thNight
34%
540
13%
1620
8%
540
39%
540
20%
1127
May11th to 20thNight
37%
531
17%
1589
10%
531
44%
531
24%
1061
May21st to endNight
48%
583
14%
1552
8%
582
52%
582
27%
1053
June1st to 10thNight
50%
540
15%
1440
10%
540
55%
540
34%
911
June11th to 20thNight
47%
540
16%
1440
11%
540
51%
540
33%
905
June21st to endNight
52%
540
16%
1440
10%
540
56%
540
33%
908
July1st to 10thNight
63%
525
17%
1400
12%
525
65%
525
40%
817
July11th to 20thNight
63%
522
20%
1392
16%
522
66%
522
39%
838
July21st to endNight
65%
591
21%
1487
16%
558
67%
558
39%
894
August1st to 10thNight
64%
537
20%
1574
15%
534
67%
534
36%
899
August11th to 20thNight
63%
534
23%
1598
18%
534
67%
534
35%
941
August21st to endNight
62%
594
22%
1944
21%
594
65%
594
32%
1119
September1st to 10thNight
58%
548
21%
1808
20%
548
63%
548
30%
1060
September11th to 20thNight
59%
720
28%
1980
21%
720
63%
720
37%
1177
September21st to endNight
49%
720
20%
1980
15%
720
51%
720
30%
1281
October1st to 10thNight
46%
720
22%
1980
17%
720
50%
720
31%
1276
October11th to 20thNight
30%
720
13%
2203
7%
720
38%
720
17%
1573
October21st to endNight
24%
792
14%
2574
9%
792
32%
792
16%
1873
November1st to 10thNight
31%
740
18%
2406
14%
740
38%
740
19%
1624
November11th to 20thNight
23%
760
11%
2470
7%
760
29%
760
12%
1886
November21st to endNight
24%
760
11%
2470
6%
760
29%
760
12%
1883
December1st to 10thNight
16%
897
4%
2607
2%
897
18%
897
7%
2246
December11th to 20thNight
15%
950
5%
3259
2%
949
19%
949
7%
2761
December21st to endNight
11%
1040
3%
4865
0%
1026
16%
1026
4%
4164

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
14%11%48%7%45%45%7%32%
78692
Night
16%12%5%8%6%3445%6%31%
81353
TransparencyDay
50%7%12%18%10%1
78393
Night
53%7%11%17%10%1
81173
SeeingDay
18%14%30%23%12%4
14885
Night
19%10%30%29%11%1
24945

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
15%21%8%12%9%45%43414%
2527
11th to 20thDay
13%17%8%15%17%7%44347%
2530
21st to endDay
23%21%10%13%9%443338%
2783
February1st to 10thDay
12%14%8%13%13%5%6%44418%
1741
11th to 20thDay
13%13%7%13%10%45%446%21%
1848
21st to endDay
15%14%7%11%7%35%45%6%22%
1570
March1st to 10thDay
13%12%5%8%8%45%5%5%7%28%
1900
11th to 20thDay
13%8%5%7%7%35%5%5%7%35%
2028
21st to endDay
18%11%5%7%6%35%447%30%
2335
April1st to 10thDay
19%13%5%7%5%3445%7%30%
2160
11th to 20thDay
17%11%36%5%3445%7%35%
2186
21st to endDay
17%10%47%5%35%45%6%34%
2340
May1st to 10thDay
17%8%36%5%35%45%8%36%
2340
11th to 20thDay
19%8%35%5%3445%7%38%
2301
21st to endDay
12%6%36%6%35%5%5%8%42%
2725
June1st to 10thDay
10%5%36%5%36%5%6%8%44%
2520
11th to 20thDay
12%6%25%5%34448%46%
2520
21st to endDay
11%5%26%5%35%5%5%10%42%
2520
July1st to 10thDay
6%5%245%45%5%7%9%50%
2520
11th to 20thDay
10%435%435%5%7%10%45%
2520
21st to endDay
8%4245%35%6%6%10%46%
2743
August1st to 10thDay
10%4245%35%6%8%9%43%
2355
11th to 20thDay
10%5%25%6%45%46%9%44%
2303
21st to endDay
10%6%25%6%345%6%9%44%
2408
September1st to 10thDay
11%6%36%5%46%5%6%7%42%
2133
11th to 20thDay
12%434435%5%6%7%46%
1980
21st to endDay
12%7%36%5%4445%8%41%
1980
October1st to 10thDay
16%7%36%7%34445%41%
1980
11th to 20thDay
23%9%46%7%5%6%5%47%24%
1753
21st to endDay
24%11%38%7%45%446%23%
1782
November1st to 10thDay
14%11%36%5%35%45%7%37%
1672
11th to 20thDay
15%17%5%10%7%345%5%6%23%
1710
21st to endDay
21%16%6%7%4345%46%24%
1710
December1st to 10thDay
14%20%9%14%8%45%435%14%
1568
11th to 20thDay
14%19%10%11%8%45%45%5%13%
1918
21st to endDay
21%25%7%11%8%3433410%
2783
January1st to 10thNight
16%21%10%13%8%3433415%
4428
11th to 20thNight
14%18%10%16%12%6%333310%
4440
21st to endNight
22%19%10%11%7%4434411%
4884
February1st to 10thNight
14%16%7%12%10%45%445%20%
2433
11th to 20thNight
13%14%6%11%7%36%5%4425%
2218
21st to endNight
16%15%49%7%45%45%6%25%
1884
March1st to 10thNight
15%10%5%7%5%3445%7%34%
2280
11th to 20thNight
13%10%35%6%3445%7%42%
2151
21st to endNight
19%9%47%6%35%45%6%34%
2192
April1st to 10thNight
16%10%5%6%5%35%446%36%
1800
11th to 20thNight
19%8%36%5%345%5%5%39%
1723
21st to endNight
18%8%35%4345%5%7%39%
1620
May1st to 10thNight
16%8%47%5%45%5%46%37%
1620
11th to 20thNight
18%6%36%4245%5%8%40%
1588
21st to endNight
12%6%37%5%35%5%5%7%42%
1551
June1st to 10thNight
10%5%345%345%6%9%45%
1440
11th to 20thNight
9%5%35%5%345%5%10%45%
1440
21st to endNight
9%5%245%45%6%6%9%45%
1440
July1st to 10thNight
7%425%5%2346%8%54%
1440
11th to 20thNight
7%414435%5%6%8%52%
1440
21st to endNight
6%425%5%25%5%6%8%52%
1575
August1st to 10thNight
9%325%4345%7%8%51%
1583
11th to 20thNight
9%4245%35%46%9%50%
1598
21st to endNight
9%5%35%5%3446%6%50%
1944
September1st to 10thNight
10%425%435%45%8%50%
1808
11th to 20thNight
12%5%24423446%53%
1980
21st to endNight
13%7%35%5%3435%8%44%
1980
October1st to 10thNight
17%6%25%42435%6%46%
1980
11th to 20thNight
21%9%36%5%34346%33%
2203
21st to endNight
25%9%36%5%34347%31%
2574
November1st to 10thNight
15%10%36%5%345%5%6%37%
2406
11th to 20thNight
16%14%6%9%7%445%7%6%24%
2470
21st to endNight
19%14%5%8%6%44446%26%
2470
December1st to 10thNight
17%18%8%12%7%45%445%15%
2607
11th to 20thNight
17%17%10%11%8%35%445%16%
3279
21st to endNight
27%21%6%9%6%34345%12%
4884

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
72%10%10%7%1
2528
11th to 20thDay
82%7%7%31
2530
21st to endDay
81%6%7%42
2783
February1st to 10thDay
69%7%7%13%4
1741
11th to 20thDay
64%6%10%13%7%
1858
21st to endDay
62%7%12%13%5%1
1570
March1st to 10thDay
55%7%13%18%7%1
1900
11th to 20thDay
48%7%13%19%11%2
2028
21st to endDay
54%6%12%19%8%1
2335
April1st to 10thDay
54%7%10%19%10%
2160
11th to 20thDay
47%6%12%19%13%2
2186
21st to endDay
48%7%13%16%14%2
2340
May1st to 10thDay
47%7%13%21%11%1
2340
11th to 20thDay
45%6%12%20%14%2
2305
21st to endDay
39%8%13%24%14%1
2725
June1st to 10thDay
36%7%13%26%15%2
2520
11th to 20thDay
36%7%12%23%20%1
2520
21st to endDay
37%9%13%22%17%2
2520
July1st to 10thDay
28%8%15%28%17%4
2451
11th to 20thDay
32%9%13%27%17%2
2445
21st to endDay
31%9%15%24%19%3
2591
August1st to 10thDay
33%9%16%28%13%2
2340
11th to 20thDay
35%9%15%26%14%2
2303
21st to endDay
34%7%15%25%16%3
2408
September1st to 10thDay
39%8%14%23%14%2
2133
11th to 20thDay
34%8%12%31%14%1
1980
21st to endDay
41%6%14%23%16%
1980
October1st to 10thDay
44%7%9%24%14%2
1980
11th to 20thDay
58%8%13%14%7%
1753
21st to endDay
61%7%13%14%5%1
1782
November1st to 10thDay
47%7%14%20%12%1
1672
11th to 20thDay
61%7%12%15%5%1
1710
21st to endDay
62%6%14%13%4
1710
December1st to 10thDay
73%7%9%8%2
1568
11th to 20thDay
71%9%10%9%1
1915
21st to endDay
77%7%7%7%1
2783
January1st to 10thNight
73%8%8%7%4
4428
11th to 20thNight
80%6%7%5%3
4440
21st to endNight
76%8%8%6%2
4884
February1st to 10thNight
67%7%9%12%5%
2433
11th to 20thNight
61%7%9%16%7%
2219
21st to endNight
59%7%12%14%7%1
1884
March1st to 10thNight
49%7%14%22%8%1
2280
11th to 20thNight
42%6%13%23%13%3
2151
21st to endNight
52%5%10%22%10%2
2192
April1st to 10thNight
49%7%11%20%13%1
1800
11th to 20thNight
47%6%9%17%18%4
1723
21st to endNight
44%6%13%18%15%3
1620
May1st to 10thNight
47%7%12%20%11%2
1620
11th to 20thNight
43%7%11%18%18%3
1589
21st to endNight
41%8%12%23%15%2
1552
June1st to 10thNight
35%6%11%29%16%3
1440
11th to 20thNight
33%9%14%24%19%2
1440
21st to endNight
34%8%16%25%15%2
1440
July1st to 10thNight
26%8%13%29%19%5%
1400
11th to 20thNight
28%8%14%28%20%2
1392
21st to endNight
29%7%14%25%20%4
1487
August1st to 10thNight
28%8%17%30%15%2
1574
11th to 20thNight
30%9%14%30%17%2
1598
21st to endNight
32%7%12%26%21%3
1944
September1st to 10thNight
32%8%14%26%17%2
1808
11th to 20thNight
30%6%12%33%17%2
1980
21st to endNight
39%6%15%26%14%1
1980
October1st to 10thNight
39%6%11%26%17%2
1980
11th to 20thNight
51%7%13%19%9%1
2203
21st to endNight
54%7%13%18%9%
2574
November1st to 10thNight
46%7%11%22%13%1
2406
11th to 20thNight
58%8%14%15%6%1
2470
21st to endNight
59%7%15%16%3
2470
December1st to 10thNight
72%7%8%9%4
2607
11th to 20thNight
70%7%11%9%3
3281
21st to endNight
74%7%10%6%2
4884

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
19%18%37%20%41
270
11th to 20thDay
23%19%41%12%4
270
21st to endDay
26%17%35%19%3
297
February1st to 10thDay
21%27%30%17%32
270
11th to 20thDay
19%31%34%15%1
270
21st to endDay
23%34%33%8%2
225
March1st to 10thDay
21%28%38%9%3
270
11th to 20thDay
22%18%32%19%9%1
398
21st to endDay
29%16%30%18%5%2
490
April1st to 10thDay
25%18%30%20%6%1
420
11th to 20thDay
27%21%26%16%6%3
412
21st to endDay
25%15%30%19%10%1
420
May1st to 10thDay
22%18%28%21%8%2
423
11th to 20thDay
19%14%29%21%12%5%
447
21st to endDay
15%11%28%26%15%4
679
June1st to 10thDay
11%10%34%29%11%4
630
11th to 20thDay
14%11%29%30%12%4
630
21st to endDay
14%8%35%26%13%4
630
July1st to 10thDay
8%6%25%31%21%9%
621
11th to 20thDay
10%9%26%30%18%7%
618
21st to endDay
10%431%31%17%7%
684
August1st to 10thDay
10%424%25%22%14%
474
11th to 20thDay
11%5%21%28%26%10%
439
21st to endDay
13%6%23%29%21%10%
495
September1st to 10thDay
14%8%22%30%18%9%
450
11th to 20thDay
10%6%20%29%26%10%
450
21st to endDay
15%12%22%26%17%8%
450
October1st to 10thDay
16%14%26%23%17%4
450
11th to 20thDay
24%19%30%23%4
359
21st to endDay
31%17%38%9%41
297
November1st to 10thDay
22%17%36%17%7%1
270
11th to 20thDay
22%27%33%13%5%
270
21st to endDay
28%16%41%12%3
270
December1st to 10thDay
24%29%28%13%6%1
270
11th to 20thDay
22%27%34%13%4
270
21st to endDay
24%28%35%11%2
297
January1st to 10thNight
21%16%33%23%6%1
950
11th to 20thNight
24%14%34%23%5%1
950
21st to endNight
22%10%34%30%4
1045
February1st to 10thNight
22%15%35%25%3
789
11th to 20thNight
20%15%35%25%4
743
21st to endNight
24%16%38%19%3
628
March1st to 10thNight
18%14%35%26%6%
760
11th to 20thNight
17%14%36%25%6%1
760
21st to endNight
22%11%32%27%7%1
757
April1st to 10thNight
19%12%35%28%6%
540
11th to 20thNight
23%10%33%26%7%1
533
21st to endNight
24%10%31%28%8%1
540
May1st to 10thNight
19%9%32%32%8%1
540
11th to 20thNight
19%8%31%30%12%
531
21st to endNight
14%5%27%34%19%1
584
June1st to 10thNight
11%6%28%40%15%1
540
11th to 20thNight
11%431%37%16%
540
21st to endNight
11%426%39%19%2
540
July1st to 10thNight
8%318%41%27%3
525
11th to 20thNight
7%221%44%23%3
522
21st to endNight
8%121%43%27%1
591
August1st to 10thNight
9%218%43%27%3
537
11th to 20thNight
10%218%41%25%4
534
21st to endNight
11%318%41%25%3
594
September1st to 10thNight
9%421%40%21%4
548
11th to 20thNight
12%419%37%24%3
720
21st to endNight
14%10%22%31%20%3
720
October1st to 10thNight
19%6%25%31%18%1
720
11th to 20thNight
26%13%26%26%8%1
720
21st to endNight
29%11%32%21%6%1
792
November1st to 10thNight
20%12%31%27%9%
740
11th to 20thNight
23%15%34%22%6%1
760
21st to endNight
25%14%31%25%4
760
December1st to 10thNight
25%15%32%23%5%
897
11th to 20thNight
25%15%36%20%41
950
21st to endNight
31%12%37%18%1
1045
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